Mucosal IgA is an important first line of defense against pathogens and may even help prevent autoimmune disease. The effectiveness of systemic immunity declines with age, leaving the elderly susceptible to life- threatening diseases. This dysregulation of the aged immune system results in poor humoral responses to T-dependent, protein antigens. Antibodies produced by aged individuals show decreased isotype switching, fewer somatic mutations, and increased autoreactivity. This predominance of "natural immunity" is accompanied by an increase in the B1 cell population. However, not all humoral responses are affected;while TD responses appear to decline, T-independent responses appear to increase. Whether or not similar effects are seen in mucosal immunity remains controversial. Here, we approach the issue of IgA producing during aging from a novel perspective - a homogeneous population of antigen-binding cells by using VDJ knock-in B cells (QM B cells). These transgenic cells retain the ability to undergo normal events associated with immunopoiesis such as isotype switching to IgA and affinity maturation. Additionally, since the cognate antigen, nitrophenyl (NP), is not found in the environment, this provides a unique opportunity to examine the effects of antigen exposure on the skewing of mucosal B cell subsets during aging. The specific aims of this proposal are 1) What are the relative contributions of B1 and B2 cells to mucosal IgA responses during aging? and 2) How does aging impact isolated lymphoid follicles (ILF)? To accomplish Aim 1, we will focus on the history of antigen exposure (both TD and TI) and its influence on mucosal IgA responses in Peyer's patches (PP) and lamina propria (LP). Using adoptive transfers, we will also determine if aged B2 cells can produce robust GC responses in PP under ideal conditions (appropriate T cell help and a young microenvironment) or if they have intrinsic defects. Finally, as B1 cell number and function increases during aging, we will examine if aged B1 cells are able to respond to TD antigens in the mucosal environment under ideal conditions also using adoptive transfers. For Aim 2, we will examine how aging impacts isolated lymphoid follicles (ILF). These dynamic structures are important sites of T cell independent IgA production and little is known about their function in aged animals. The results of this study will clarify the role of B cell subsets in mucosal IgA production in aged individuals and help future clinical efforts to increase protective immunity in the elderly. PUBLIC HEALTH RELEVANCE: The project described in this proposal examines the impact of advancing age on mucosal immune function. Elderly are at increased risk of gastrointestional infections and respond poorly to oral immunization. By better understanding the changes behind the aged immune system, particularly antibodies produced by white blood cells, scientists and physicians can design better therapies and interventions.